Microporous materials have varied applications in the fields of separations, analytics and absorption. Filtration is the most fully developed of these arts and provides some technical background for the instant invention.
Microporous materials are characterized by their filtrate permeability (flow rate), minimum particulate retention size (pore size), continued permeability as particulates are collected, (clogging resistance), strength, dimensional stability, and wettability.
The flow rate of a porous material is a measurement of volume of fluid passage per unit time for a given depth and area of the material at a known differential pressure. Flow rate is influenced by the size, internal physical structure and distribution of the pores; for a given material area, depth and maximum pore size (measured by bubble point or particulate challenge), lower flow rates indicate fewer pores, a greater portion of smaller sized pores, or both.
Pore size may be characterized by the bubble point, .sup.1 a measurement of the gas pressure required to remove a liquid from the pores of a saturated material. Bubble point measurements are typically reported for the removal of water by air, a standard of comparison now recognized in the microfiltration industry. If, for example, a filter formed from microporous material exhibits a bubble point of 6 psi and a flow rate of 13 cc/min cm.sup.2 at 10 psi, then the relationship of flow rate and bubble point is not good relative to commercial filters. This bubble point is indicative of a pore size of around 5.0 um, whereas the flow rate is indicative of a 0.2 um pore size. This condition produces a membrane of poor filtration performance, having a comparatively low flow rate for the maximum pore size of 5 um. FNT 1 American Society for Testing and Materials, Standard F316; and Brock, Thomas D., Membrane Filtration: A User's Guide and Reference Manual, Science Tech, Inc., Madison, Wis., 1983.
Pore size can also be measured by particulate challenge, the process of checking a material's particulate retention for progressively smaller particulates of known dimensions. Gradients of particulate size are commercially available and detection of particulate passage through the material is possible by a variety of analytical techniques, such as light scattering analysis, turbidity analysis, or subsequent smaller pore filtration and particle counts. Particulate challenge is generally more accurate and convenient for pore sizes greater than 50 um than are bubble point determinations.
Clogging resistance is measured by passage or absorption of successive aliquots of a solution containing particulates retained by the material, then comparing the passage or absorption time of the successive aliquots. Commercial scale filtration of microscopic particulates has long been complicated by filters formed from materials which rapidly clog, and thus require frequent cleaning or replacement, thereby limiting filtration capacity.
Strength and dimensional stability of porous materials determine the manufacturing tolerances required for successful formation of commercial grade products, the differential pressure which the materials can withstand, as well as resistance to mechanical tear. For example, in filtration applications swelling and growth of unsupported nylon materials due to solvent absorption can cause mechanical problems.
Wettability is a material's propensity to absorb a particular solvent, and can be measured by the weight percentage of solvent absorbed at saturation, or by the time required for dry materials to reach saturation upon direct contact with the solvent ("wet out time"). Because some porous nylon materials are reported to exhibit reduced wettability at or near their melting points. See Pall, U.S. Pat. No. 4,340,479, saturation capacity and saturation time are generally considered as specific to temperature ranges. While the performance of porous materials with low wettability can be improved by pre-wetting with low surface tension solvents, followed by flushing with the target solvent immediately prior to use, pre-wetting is not only expensive due to cost of the additional solvent, but also due to the additional steps required to use the material for its intended purpose.
Because porous materials are often formed in sheets, fabrication of commercial products often requires sealing multiple surfaces of the material in order to form desirable configurations, for example, filtration cartridges. Seals formed by heating and then placing in contact the affected surfaces are inexpensive and do not require the use of adhesives which might contaminate the filtrate. Thermally formed seals however often reduce the wettability of porous nylon materials.
Several filters have been synthesized from nylon polymer materials. Paine's U.S. Pat. No. 3,408,315 describes a membrane made by first dissolving nylon polymer in a solvent or solvents, forming a base solution and then adding other miscible reagent mixtures to the base solution. A thickening agent such as Cab-O-Sil fumed silica is added to increase the viscosity of the solution to the range of 500-1000 centipiose (Brookfield). Paine's nylon terpolymer solution is then metered onto a belt and the solvents are evaporated, causing precipitation of the nylon into a thin film porous structure. This is known as an air casting process because air is used as the fluid to carry away the solvents from the film of polymer solution.
Paine membranes have limited utility due to their solubility in alcohol and many other solvents. Film membranes produced by this casting method have relatively weak tear and puncture resistance and undergo problematic dimensional changes when wetted with water or on drying after being wet with water.
Marinaccio et al.'s U.S. Pat. No. 3,876,738 describes a process for producing alcohol insoluble microporous membranes of nylon polymers using a wet casting process in which a liquid in contact with the cast film serves to remove the solvent from the polymer containing solution. The Marinaccio process involves metering a controlled thickness of polymer solution onto a drum which is partially longitudinally immersed in a bath containing a non-solvent for the polymer. As the drum rotates the cast film is immersed into the bath. As the solvent is extracted by the bath, the polymer precipitates as a film on the drum; removal of the film and subsequent processing are determined by the intended end use of the film.
Pore formation in Marinaccio's method is dependent upon polymer concentration, the solvent system used to make the polymer solution, the age of the polymer solution, composition of the solvent extraction bath, bath temperature and additives to the mix or the bath. Marinaccio explains that the pore size would increase slightly with polymer concentration because of the increasing aggregation tendency at higher concentrations because the more polymer in solution, the longer the chains of these polymers, and hence the larger spherical shape they will form when precipitated. These aggregates are Marinaccio's means of controlling pore sizes and these aggregation tendencies are modified by using various ratios of non-solvent to solvent, thereby changing the overall solvent power of the solution.
Marinaccio's film membranes suffer from relatively low tear resistance and strength, and dimensional instability. Because nylon polymers readily absorb up to eight weight percent water, the films swell and grow in size, causing problems in filtration and other uses.
Pall's U.S. Pat. No. 4,340,479 claims a skinless, microporous, hydrophilic polyamide membrane produced from alcohol insoluble, hydrophobic nylon polymers, a phenomenon which he claims only occurs with ". . . ratios of CH.sub.2 (methylene) to NHCO (amide) within the range from about 5:1 to 7:1." (Column 8, line 24), thus teaching away from the use of polyamide polymers with a CH.sub.2 to NHCO ratio outside the range of 5:1 to 7:1 when synthesizing hydrophilic membranes.
Pall's method begins with nylon 66 resin dissolved in a solvent by a mixing regimen. To this starting solution another solution or non-solvent blend is added to create a visible precipitate of polymer, a necessary stage which results in what Pall terms "nucleation" or a "state of nucleation". The precipitate is then totally or partially redissolved to form a casting solution which is formed into a thin film on a substrate and with minimum delay is immersed in a bath which is comprised of a nonsolvent for the polymer and a solvent for the starting solvent. Pall states that many factors influence the nucleation state of the casting solution and hence the final filter properties. These include the temperature of the starting polymer and solvent; the rate and intensity of mixing by which the non-solvent mixture is added; and, the geometry of the mixing vessel.
Pall describes alcohol-insoluble polyamide resins as inherently "hydrophobic," and claims a method of producing from these resins membranes which are "hydrophilic" unless heated to near melting where they revert to the "hydrophobic" state. Because the near melting point temperatures used to effect thermal seals can result in a reversion of these membranes to a "hydrophobic" state, Pall membranes may not be wettable in the seal area with high surface tension solvents such as water and therefore difficult to test and problematical in application.